Genuine U.S. currency has a magnetic property which is capable of being detected by a magnetic sensor. A hysteresis curve in which magnetization is plotted against magnetizing force of the black ink on a U.S. one dollar bill is shown in FIG. 1. The plot is in units of EMU and Oersteds. The plot of FIG. 1 shows that the retentivity of black ink on U.S. paper currency is significantly less than the saturation level, indicating that it is preferable to sense the saturation level rather than the retentivity level. This can be accomplished by magnetizing the bill at the sensing location.
Previous attempts to accomplish this suffer from the following problems:
1. The magnet tends to saturate the mu-metal shield surrounding the magnetic pick up head, thereby reducing the shielding effectiveness, and
2. The magnet tends to vibrate during machine operation causing an unacceptable magnetic noise source.